Our overall objective is to characterize endogenous guinea pig viruses (GPV) and determine their role in leukemia. We have previously shown that endogenous viruses can be induced by bromodeoxyuridine and iododeoxyuridine treatment of cultured cells. We have further show that proviral DNA is present in all guinea pig cells induced or not and viral induction is due to the activation of proviral DNA. We have determined the state of proviral DNA in guinea pig cells, we have shown that part of the viral DNA homologous to one-third of the viral genome is repeated 6-10 times, whereas the rest of the proviral DNA representing two-thirds of viral genome is present only 1-2 times per haploid guinea pig cell. We have recently found that BUdR induced viral genes are fully expressed in leukemic guinea pig cells. We are now characterizing the viral RNA species in BUdR induced and in leukemic cells. BIBLIOGRAPHIC REFERENCES: Nayak, D.P., and Davis, A.R. The nature of endogenous oncornaviral DNA sequences in mammalian cells. VII International Symposium on Comparative Leukemia Research. Copenhagen, Denmark (1976). Nayak, D.P., and Davis, A.R. Endogenous guinea pig retravirus and L2C leukemia. Presented at the Symposium on Guinea Pig L2C Leukemia: Immunological, Virological and Clinical Aspects. Bethesda, Maryland (1976) Federation Proceeding (1977) in press.